And I'm Renee Montagne. Senators on the Foreign Relations Committee approved military force in Syria after first trying to make sure they didn't give away too much. Lawmakers sent the Senate a resolution after struggling to reconcile competing goals. Senators wanted to narrow the U.S. commitment in Syria, yet they also wanted to avoid tying the hands of the Commander-in-Chief. NPR's Larry Abramson reports.

LARRY ABRAMSON, BYLINE: Without question, lawmakers' biggest fear is that U.S. troops could be coming home in body bags from Syria. Before a House hearing yesterday, Secretary of State John Kerry was at pains to convince lawmakers, the president does not want to see any U.S. casualties.

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SECRETARY OF STATE JOHN KERRY: We all agree, there will be no American boots on the ground.

ABRAMSON: But promises may not be enough. Some members, like Florida Congresswoman Ileana Ros-Lehtinen, said she wants tighter language than the Senate version, which only prohibits the use of U.S. troops in a combat role.

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REPRESENTATIVE ILEANA ROS-LEHTINEN: This sounds like it leaves open the possibility of boots on the ground for something other than combat, like Special Operations.

ABRAMSON: The House may well insist on tougher language barring the use of troops. That brings up a key tension for Congress. Can lawmakers rein in the president, without limiting the military's effectiveness on an unpredictable battlefield? Even on the issue of boots on the ground, the Senate version leaves the president some wiggle room. Cully Stimson of the Heritage Foundation says, the president may still claim the power to use troops once the battle begins.

CULLY STIMSON: In other words, we're not authorizing them, wink wink, nod nod, but of course we realize and recognize that you have the constitutional authority to put them in harm's way.

ABRAMSON: Lawmakers are also trying to set a time limit on U.S. involvement - that's to avoid a drawn out conflict. Republican Congressman Chris Smith of New Jersey recalls what happened in 1999, when the U.S. participated in the NATO bombing of Serbia during the war over Kosovo. Even though Congress did not approve that action, President Clinton had promised lawmakers a short campaign.

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REPRESENTATIVE CHRIS SMITH: And I know many people had thought, including in Brussels at NATO headquarters, it would last just a few days. It lasted 78 days.

ABRAMSON: To prevent a repeat, the Senate version would limit the campaign to 60 days, with a possible 30 day extension, something the White House appears willing to accept. In building these restrictions, Congress has many negative experiences to draw from: the wars in Iraq and Afghanistan, of course, went on much longer than planned.

And Congress is still smarting from the 2001 authorization of military force that launched the war on terror. That broadly worded document is still in effect, and it has been used to justify drone strikes that have killed thousands, from Pakistan to Yemen. The question is, according to George Mason University Law Professor Ilya Somin, what happens if the mission in Syria isn't done in 90 days.

ILYA SOMIN: That, I think, is a more realistic and perhaps more dangerous scenario where the president could clash with Congress but it would be a very substantial risk for him to do so in that way.

ABRAMSON: But professor Ilya Somin says even without these congressional limits, the White House is likely to be cautious about extending this campaign. That's because, as lawmakers emphasized during this week's hearings, polling shows there's very limited popular support for the attacks on Syria. Larry Abramson, NPR News, Washington. Transcript provided by NPR, Copyright NPR.